The optical properties of metallic nanoparticles and hole arrays continue to draw interest because of their fundamental science and potential for emerging applications. For example, such structures can be used over a range of wavelengths from the ultraviolet to the near-infrared in the research and development of nanoscale photonics, chemical and biological sensing, high-efficiency photovoltaic devices and related applications. Electromagnetic radiation can interact with metallic nanoparticles through the resonant excitation of their surface free electrons. These collective electron density oscillations, known as surface plasmons (SPs), are confined to a finite volume, and are sensitive to the size, shape and dielectric environment of a nanoparticle or hole structure.
With regard to metallic particles, both solution-based syntheses and fabrication techniques have been used in the art. Most research efforts have focused on the chemical synthesis of nanoparticles because the preparative techniques are relatively straightforward; the size and shape of nanoparticles can be tailored by controlling conditions such as reaction temperature, surfactants, and concentrations of precursors. Spherical particles with sizes less than 50 nm support single LSP resonances that are dipolar in character. Accordingly, their optical properties can be explained reasonably well by the lowest order term in Mie theory. In contrast, larger metallic particles (diameters >100 nm) with anisotropic shapes can exhibit multiple LSP resonances that correspond to higher order modes. Disordered assemblies of 100-nm Ag particles imbedded in poly(dimethylsiloxane) (PDMS) films showed a dipole resonance as well as a quadrupole resonance as the film was stretched in two-dimensions. Solution-phase synthesis often provides a flexible route to size and shape control of metallic nanoparticles, which can in turn provide tuning and control of their respective optical properties. However, a common drawback of such solution-based preparations, especially with regard to synthesis of noble metal nanoparticles, is the formation of various other shapes in addition to the desired particulate configuration.
Sacrificial templates have been employed in the art for molding the size and shape of free-standing nano- and mesostructures. Typically, solid and supported structures, such as nm-sized pores in anodized alumina membranes or μm-sized etched pits in silicon are used. Electro-deposition of conducting materials or molding of polymers reproduces the shape and structure of the template; the templates are then removed by the appropriate etchants. Free-standing structures such as metallic (and multi-layered) rods, pyramidal tips for scanning probe applications, and μm-sized metallic pyramidal shells have been produced. The μm-sized pyramidal shells, for instance, were found to exhibit tips with radius of curvature r as small as 50 nm. Other types of templates, including silica spheres, have recently been used to fabricate metallic structures with unusual shapes. Such structures were generated by e-beam deposition of metal onto silica spheres followed by etching of the sphere-template. Sub-micron “half-shells” made from different metals as well as “crescent moon” structures with sharp edges in silver were also produced. The edges of these silver shell-structures enhanced the local electromagnetic field, and the Raman scattering of Rhodamine 6G from isolated, individual crescent moon structures could be detected.
Investigations of sub-micron particles have only recently been possible through improved chemical methods and fabrication techniques to generate particles with uniform size and shape. Although multipolar LSP resonances have been seen in the extinction spectra of sub-micron particles, the random dispersion of the particles in solution ensured that all resonant plasmon modes were measured simultaneously, and some peaks were obscured because of polarization averaging. Multipolar excitations can, however, depend on the direction of the propagation wavevector and polarization vector; thus, certain excitation angles can make selected resonances more pronounced. To correlate the orientation of the particles with specific plasmon modes directly, strategies that can both isolate particles and control their orientation are essential. Drop-coating or spin-casting dilute colloidal solutions onto glass slides have resulted in isolated particles although their orientation on the substrate was not well-defined, and the optical properties could only be related to particle shape in a non-uniform dielectric environment. Electron-beam lithography can create individual particles with a controlled orientation, although the shapes are limited to two-dimensional planar structures.
The observation of enhanced transmission through subwavelength hole arrays has also generated considerable interest. Such arrays make possible new fundamental studies of SP interactions with periodic structures and novel technologies including spectroscopically based chemical and biological sensors and photonic devices. To date, the most common method to fabricate hole arrays is focused ion beam (FIB) milling, a serial and low throughput approach that can control the diameter and spacing of the holes with reasonable precision. Free-standing suspended films have been fabricated by FIB and reactive ion etching, but the generation of multi-layered films is challenging and laborious and has been limited to only a few metals. No technique has been developed for producing optical quality hole arrays in a parallel fashion out of multiple materials and certainly not in areas larger than hundreds of square microns. Increased access to and expanded capabilities of these nanostructured films are critical for in practical photonic devices and in biological and chemical sensing applications. As with nanoparticles, there is an on-going search in the art for a facile, efficient method of preparing suitable hole array structures.
The transmission of light through films of nanohole arrays depends strongly on the shape of the hole because only wavelengths of light that are resonant with the SP can be transmitted through hole arrays with appreciable intensity. Most studies to date have focused on the optical properties of circular or slightly oblong hole arrays on small pitches in Ag films fabricated by FIB. (See, Ebbesen et al. Nature 2003, 424, 824-830; and Ebbesen et al. Adv. Mater. 1999, 11, 860-862.)
Holes, compared with particles, offer unique opportunities to investigate the effects of polarization on their localized surface plasmon resonance (LSPR). For example, several reports showed shape-dependent enhanced transmission through hole arrays with different polarization of incident light. Rectangular hole arrays exhibited more dependence on polarized light and increased transmission compared to circular or square hole arrays. It is anticipated that nanoholes and nanoparticles are complementary structures, although most work to date has focused only on circular holes and disks. FIB can control the shape of the holes with reasonable precision; however, this serial process cannot easily generate free-standing films or hole shapes having sharp corners or edges of high curvature. Again, the art continues the search for facile fabrication of hole arrays over a range of hole shapes and configurations.